Fluctuating asymmetry of red deer antlers negatively relates to individual condition and proximity to prime age

After about 50 years of research on fluctuating asymmetry (FA) as a reliable indicator of both individual quality and environmental stress, the enthusiasm is beginning to decline. The findings of many studies are inconsistent and the relationship between FA and stress appears both weaker and more complex than first thought. To provide clarification of the debate, new studies should use more efficient and unified statistical protocols, large sample sizes and joint analysis of several related traits. In addition, fitness–FA associations should be tested at the individual level, in different populations and under different environmental conditions. To achieve these criteria, we describe a 9-year study in which we measured six antler traits of 3 000 Iberian red deer, Cervus elaphus hispanicus, from three study areas in southwest Spain. Males were harvested during hunting activities and measured, weighed and aged post mortem. We found evidence of correlations between traits in FA, an association between asymmetry and stress conditions and a weak but significant negative relationship between FA and fitness surrogates (body mass and antler size), thus supporting some assumptions of the FA hypothesis. As also predicted by theory, antler traits of less functional importance were more asymmetric and more sensitive to stress than those directly used in fighting behaviour. The relationship between age and antler asymmetry was U-shaped, suggesting an effect of sexual selection on antler development in favour of larger and more symmetrical antlers during prime age.